In a touch panel display to be mounted on an electronic device, for example, a smartphone or a tablet PC, a tempered glass sheet that has been chemically tempered has hitherto been used as a cover glass.
Such tempered glass sheet is generally manufactured by chemically treating a glass sheet containing an alkali metal as a composition with a tempering liquid to form a compressive stress layer on a surface. The tempered glass sheet includes the compressive stress layer on a main surface, and hence has improved resistance to impact on the main surface. Meanwhile, in the tempered glass sheet, a tensile stress layer is formed in conformity with the compressive stress layer on the main surface. Then, there has been a problem of breakage (so-called self-destruction) caused by extension of a crack on an end surface due to the tensile stress. Further, there has been a problem in that, when the compressive stress layer is formed to be thin over the glass sheet in order to decrease the tensile stress, sufficient impact resistance is not obtained on the end surface.
In order to solve the above-mentioned problem, there has been developed a technology involving appropriately setting the balance in compressive stress between the main surface and the end surface of the tempered glass sheet, to thereby reduce an internal tensile stress within an appropriate range. For example, in Patent Literature 1, there is disclosed a technology involving forming a film on the main surface in advance to suppress the degree of progress of chemical tempering from the end surface, to thereby control the depth of the compressive stress layer of the main surface to reduce the internal tensile stress without reducing the compressive stress layer of the end surface.